400 



MINERALOGY. 



Geognosy. 



Kail of 



mountain 



masses. 



Bursting 

 of lakes. 



Effects of 

 the ocean. 



crust of the earth ; some by the unequal deposition of 

 formations, and others by the widening of great rents. 

 It is also observed, that numerous rents and fissures, 

 and the fall of great masses of mountains, take place 

 during floods or wet seasons. These falls are oc- 

 casioned either by the weight of the masses being in- 

 creased by the great quantity of absorbed water, or by 

 the diminished cohesion of the parts of the rock effect- 

 ed by the same cause, or by the splitting of great mass- 

 es by freezing of water, or any other power that inter- 

 rupts the continuity of the rock, and favours its sepa- 

 ration into different masses. The fall of rocks is also 

 occasioned by the softening and removal of subjacent 

 strata or beds by means of water. 



These masses sometimes interrupt the course of ri- 

 vers, and thus form lakes. These lakes in their turn 

 again force a passage through this inclosing barrier, and 

 sometimes so suddenly as to deluge and desolate the 

 lower country. 



In the year 1618, the once considerable town of 

 Plurs, in Graubunden, with the neighbouring village 

 of Schilano, were overwhelmed by an immense moun- 

 tain mass, which separated with a frightful noise from 

 the south side of the mountain of Corto. In 1678, the 

 sulking of several great masses of rocks in the Pyrenees, 

 caused very violent overflowings in Gascony. In 1714, 

 the west side of the Diableret, in the Vallais, fell down, 

 and covered the neighbouring country with its ruins 

 for two miles in length and breadth, and the immense 

 blocks of stone interrupted the course of the rivers ; 

 and lakes were thus formed. Many similar instances 

 are on record in France, Italy, England, Scotland, &c. 

 When the barriers of lakes, in high and mountainous 

 countries, burst during great thaws or uncommon floods 

 of rain, tremendous overflowings of the lower country 

 that almost exceed conception are the consequence. Im- 

 mense masses of rock are torn away and carried to an 

 inconceivable distance, and whole countries are deso- 

 lated. Many of the deluges mentioned by ancient wri- 

 ters appear to have been caused by the bursting of lakes. 

 The Waller Lake, in the Tyrol, rose so much, ow- 

 ing to the melting of the ice of the neighbouring gla- 

 ciers, that it broke through its natural barrier ; and its 

 water was precipitated with so much velocity and vio- 

 lence into the lower country, that whole valleys and 

 plains were desolated. The bursting of a lake in the 

 valley of Bagne, occasioned great devastation. There 

 are many other instances of lakes having emptied them- 

 selves in other parts of Switzerland, as also in the Fich- 

 telgebirge, &c. 



The waters of the ocean also act very powerfully in 

 breaking down the land. Its waves and currents are 

 particularly active in these destroying operations. They 

 either hollow out the rocks on the coast into caves of 

 greater or less magnitude, or, by washing away softer 

 subjacent strata, cause sinkings and fallings of great 

 masses of rock. The caves in the islands of Arranand 

 Jura have been partly formed in this manner. 



If many streams act in different directions on the 

 same coast, or in conjunction with land-floods, as is of- 

 ten the case, the destroying effect is very great. Fre- 

 quently also the power of the flood is increased by ebb 

 and flood-tide. In this manner many maritime coun- 

 tries have been overwhelmed by the sea. 



The Baltic Sea affords examples of these destroying 

 effects ; thus the island of Rugen was formerly joined 

 to the Continent, but, by the violent action of the sea, 

 has been much diminished in magnitude, and separated 

 from it. The effects it has produced on the coasts of 



Carniola, Dalmatia and Egypt, are well known. The Geognosy. 

 Zuyder-zee, which is contained between the provinces S ~"S~~'' 

 of Holland, Utrecht, Gelders, Overyssel, and Friesland, 

 was formerly a lake, through which an arm of the 

 Rhine, named the Flevo, flowed towards the ocean. In 

 the 13th century the sea broke in, covered the whole 

 country, and left only detached portions of the land, 

 which now form the islands denominated Texel, Vlie- 

 land, Schelling, Newland, and others, that now serve 

 to defend this sea from the power of the waves of the 

 ocean. This remarkable change is supposed to have 

 been occasioned by a violent land-flood, in conjunction 

 with high tides, and a high wind blowing in an oppo- 

 site direction to the course of the river. 



In the year 1 1 6'4, a violent storm of wind caused a 

 deluge in Friesland, which occasioned the death of 

 many thousand persons. In 1218, another inundation 

 in Friesland occurred, by which 100,000 persons were 

 drowned ; and another equally destructive took place 

 in 1530. Floras speaks of a rising of the water in the 

 year of Rome G44, which forced the Teutoniaus, Cim- 

 brians, and Tigurians, back from the countries they in- 

 habited. This was occasioned by a violent north wind, 

 which raised the ocean along the coast of the countries 

 they inhabited. 



Water in the state of ice, also produces considerable Effects of 

 changes on the surface of the earth. Thus we often lce and 

 observe masses from a hundred-weight to many tons sn 

 floated by rivers during thaw-floods, and these frequent- 

 ly break up the banks of the rivers, and even tear away 

 immense masses of solid rock. Sea-ice also produces 

 similar effects on coasts, but on a greater scale. 



It sometimes happens, that great fields of ice rise 

 from the bottom, and bring with them masses of rock 

 several hundred tons weight. These masses of stone 

 are embedded in the ice. They are carried along with 

 the ice, and deposited on shores at a great distance 

 from their original situation. This fact will serve to 

 explain the appearance of loose blocks of particular 

 kinds, in situations far removed from their original re- 

 pository. 



Similar changes are occasioned by the fall of ice from 

 the heights of mountains. When the glacier, or moun- 

 tain ice, rests on inclined planes, dreadful devastation 

 is caused by it during the time of floods, as it then 

 splits, and is hurled down to the lower country with ir- 

 resistible impetuosity. The inhabitants of the Alps of 

 Switzerland and Savoy, of Iceland and Greenland, of- 

 ten experience the terrible effects of the fall of these 

 tremendous masses. 



In like manner, the fall of snow produces striking 

 effects. The consolidated snow is often precipitated 

 with great velocity, accompanied with terrible noises, 

 carrying along with it rocks of vast size, and often bu- 

 rying villages under it. 



The freezing of water contained in the fissures of 

 rocks also occasions considerable alterations on the sur- 

 face of the earth. This is observed most particularly 

 in those rocks that have perpendicular fissures, because 

 these allow the water to enter more easily, and favour 

 the separation of the masses when the water expands 

 during the process of freezing. Hence we find no spe. 

 cies of rock more changed by the effects of frost than 

 basalt and porphyry-slate. 



The Chemical effects of water, particularly the destroy- Chemical 

 ing effects, depend on the kind of rock overwhich it flows; destroying 

 for some allow water to act on them chemically, others cffects <& 

 do not. Limestone, gypsum, and rock.salt, are more Wl 

 particularly acted on by water than most other rocks. 



